1. Field of the Invention
The present invention relates to permanent magnet type magnetic field generating apparatuses.
2. Description of Related Art
As will be explained in detail below, generally, a dipole ring magnetic field generating apparatus comprises a plurality of permanent magnets disposed in a ring shape such that the magnetization direction of the permanent magnet pieces undergoes one rotation over a half-circumference of the ring, and generates a substantially unidirectional magnetic field in a space within the ring, and have permanent magnet pieces whose magnetic strength is the same. Such dipole ring magnetic field generating apparatuses are utilized widely as uniform magnetic field generating means, for example, in magnetic resonance imaging (MRI) devices, semiconductor element manufacturing processes and in fundamental research. (See Halbach, K. Design of permanent magnet multipole magnets with oriented rare earth cobalt material, Nuclear instruments and Methods, vol. 169, 1980, pp 1-10, which is herein incorporated by reference.) Conventionally, magnets such as normal electromagnets and super-conducting electromagnets have been used as means for generating a magnetic field that is uniaxially uniform, however with recent developments of high performance rare earth permanent magnets, it has become commonplace to use rare earth permanent magnets (referred to below simply as “permanent magnets”) as uniform magnetic field generating apparatuses for low magnetic fields, such as less than 1 T (Tesla: kg·s−2·A−1).
Conventional dipole ring magnetic field generating apparatuses and the permanent magnet pieces used therein and others are described with reference to FIG. 8. FIG. 8 is a schematic, cross-sectional view of a conventional dipole ring magnetic field generating apparatus on a plane perpendicular to its center axis. As shown in the diagram, a dipole ring magnetic field generating apparatus 1 includes a plurality of permanent magnet pieces 101 to 124 disposed in a ring shape, and preferably is encompassed by a perimeter yoke 2 on its outer side.
The permanent magnet pieces 101 to 124 that form the constituent magnet of the dipole ring magnetic field generating apparatus have substantially the same magnetic strength, and the permanent magnet pieces are magnetized in a predetermined orientation with respect to the radial direction respectively, such that when the permanent magnet pieces are disposed in a ring shape, the magnetization direction of the permanent magnet pieces undergoes one rotation over a half-circumference of the ring. Thus, the permanent magnet pieces are magnetized so that permanent magnet pieces that are directly opposite when viewed from the center axis of the ring that constitutes the magnetic field generating apparatus (for example permanent magnet pieces 101 and 113) have a difference in magnetization direction of 180 degrees. More specifically, it is preferable that the permanent magnet pieces are magnetized at the angular shown in formulas (1) and (2) below. However, in accordance with such factors as conditions of use and optimization, it is also possible to arrange and adjust the magnetization direction within a range of, for example, ±about 5°.
                                                                        θ                ⁢                                                                  ⁢                n                            =                                                -                                      360                    N                                                  *                n                                                                                        (                                                      n                    =                    1                                    ,                  2                  ,                  …                  ⁢                                                                          ,                                      N                    /                    2                                                  )                            ⁢                                                                                                      (        1        )                                                                                    θ                ⁢                                                                  ⁢                n                            =                              360                ⁢                                  (                                                            n                      N                                        -                    1                                    )                                                                                        (                                                n                  =                                                            N                      /                      2                                        +                    1                                                  ,                                                      N                    /                    2                                    +                  2                                ,                …                ⁢                                                                  ,                N                            )                                                          (        2        )                N: number of divisions of the constituent magnet (natural number).    n: permanent magnet piece number (natural number).    θn: magnetizing direction of the nth permanent magnet piece.
By such a configuration, a magnetic field (main magnetic field component) that is substantially unidirectional, and that is of substantially uniform strength is generated in the space inside the ring of the dipole ring magnetic field generating apparatus 1. In the following, the Z-axis is defined as a center axis of the ring that constitutes the dipole ring magnetic field generating apparatus, the Y-axis (the NS magnetic field direction in FIG. 8) is defined as an axis that passes through the center of the ring and that is parallel to the unidirectional magnetic field and the X-axis (the EW magnetic field direction in FIG. 8) is defined as an axis that passes through the center of the ring and that is perpendicular to the Z-axis and the Y-axis. Here, the center of the ring that constitutes the magnetic field generating apparatus is on the center axis of the ring, and the mid point in the center axis direction.
As explained above, one of the characteristics of the dipole ring magnetic field generating apparatus is that the magnetic field of substantially a single direction can be formed in the inner space of the magnetic field generating apparatus. When the NS direction (the Y-axis direction) of the magnetic field generated in the inner space of the dipole ring magnetic field generating apparatus is 0°, the angle of the magnetic field vector at points in the inner space (referred to below as “skew angle”) is substantially 0° in the vicinity of the center axis of the ring, but due to the properties of the magnetic field generating apparatus, it tends to deteriorate, namely become large, as it approaches the inside wall of the magnetic field generating apparatus.
During use of the ordinary ring magnetic field generating apparatuses, the magnetic field components in which this skew angle is large are generally seen as impurities, that is to say, as noise. Particularly the skew angle component in the XY-plane of the inner space of a magnetic field generating apparatus such as shown in FIG. 8 may have a large influence on the performance of elements manufactured in the manufacturing process of, for example, substrates for semiconductors, and there is a need to make the skew as small as possible.
Furthermore, as noted above, another characteristic of a dipole ring magnetic field generating apparatus is that the magnetic field of substantially uniform strength can be formed in the inner space of the magnetic field generating apparatus, that is to say that the uniformity of the magnetic field is favorable. These two characteristics, as well as the quality of the magnetic efficiency, are the biggest advantages of this magnetic field generating apparatus, and they are the reason for the wide industrial utilization of dipole ring magnetic field generating apparatuses. However, improving the skew angle and improving the uniformity of the magnetic field cannot always be achieved at the same time, but rather there is the risk that if one is optimized, characteristics of the other may be lost.